In many electronic applications, electrical resonators are used. For example, in many wireless communications devices, radio frequency (rf) and microwave frequency resonators are used as filters to improve reception and transmission of signals. Filters typically include inductors and capacitors, and more recently resonators.
As will be appreciated, it is desirable to reduce the size of components of electronic devices. Many known filter technologies present a barrier to overall system miniaturization. With the need to reduce component size, a class of resonators based on the piezoelectric effect has emerged. In piezoelectric-based resonators, acoustic resonant modes are generated in the piezoelectric material. These acoustic waves are converted into electrical waves for use in electrical applications.
Various types of filters use mechanical resonators, such as bulk acoustic wave (BAW) resonators, including film bulk acoustic resonators (FBARs) and solidly mounted resonators (SMRs), or surface acoustic wave (SAW) resonators. The resonators convert electrical signals to mechanical signals or vibrations, and/or mechanical signals or vibrations to electrical signals. A BAW resonator, for example, is an acoustic device comprising a stack that generally includes a layer of piezoelectric material between two electrodes. Acoustic waves achieve resonance across the acoustic stack, with the resonant frequency of the waves being determined by the materials in the acoustic stack and the thickness of each layer (e.g., piezoelectric layer and electrode layers).
Desirably, the BAW resonator excites only thickness-extensional (TE) modes, which are longitudinal mechanical waves having propagation (k) vectors in the direction of propagation. The TE modes desirably travel in the direction of the thickness (e.g., z-direction) of the piezoelectric layer.
In general, the most important vibrational mode for radio frequency (RF) filter applications is the TE mode, which is in parallel with an electrical field or perpendicular to the BAW resonator surface. The longitudinal mode is activated by varying electrical voltage across the BAW resonator, and therefore the electrical field across polarized charges (i.e., dipoles, consisting of positive and negative charged ions in AlN film), resulting in contraction and expanding dependent on the direction of the electrical field. At a certain frequency, vibration of the dipoles is in phase with the electrical field, where series resonance occurs and its correspondent frequency is called series resonant frequency (Fs). Where the vibration is totally out of phase with the electrical field (i.e., 180° phase difference between the phases of the vibration and the electric field), the resonator reaches to parallel resonance, and its corresponding frequency is called parallel resonant frequency, (Fp).
In known BAW resonators, loss of acoustic energy at the interfaces of the BAW resonator ultimately degrades the electrical performance of the BAW resonator and devices that comprise such BAW resonators.
What is needed, therefore, is a BAW resonator that overcomes at least the shortcomings of known BAW resonators described above.